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Network Layer and Routing
NL responsibility
• Main responsibilities
– Host to Host delivery.
– Finding path to destination.
• Services provided to TL, receive from DL
– Connectionless and connection oriented services
– Identifying source and destination uniquely and thereby use NL
address. Fragments TL data if necessary. Uses packet switching
(store and forward) with datagram approach.
– In the router NL finds the appropriate interface from which it will
reach the destination.
– At the destination matches the address and check corrupt
packets, and assembly fragmented units and then sent to TL
Switching
• How is a packet sent from one node to
another node.
– Circuit switching
– Packet switching
• VC approach – call set up before message transfer
• Datagram approach – independent packet, does
not need call setup phase.
Internet Protocol
• Network layer of datagram-oriented network
such as internet has three major components:
– Network protocol, e.g IP
Transport Layer:TCP, UDP
– Path determination
Protocols:
IP Protocols:
– Error reporting Routing
• path selection
•RIP, OSPF, BGP
Routing
Table
• addressing conventions
•Datagram format
•Packet handling conventions
ICMP protocol:
• error reporting
•Router “signaling”
Link Layer
Internet structure
• At the NL internet can be viewed as a
collection of subnets or Autonomous systems
(AS) that are interconnected.
Internet working
• TL takes data streams and breaks them up
into datagrams. Datagrams cab be up to
64Kbytes each, but usually not more than
1500 bytes. If necessary the fragments
them and sent to internet. When all pieces
finally reaches the destination they are
reassembled by NL to original datagram. It
is then handed over to TL which inserts it
into receiving process.
IPv4 addressing
• IPv4 is 32 bits long
• Usually written in dotted
decimal notation, eg.
193.32.216.9
• In the globak internet
each interface must have
a unique IP address.
• Network address and
host address.
• Subnet mask
• Meaning of 223.1.1.0/24
IPv4 header
4 bits
4 bits
Version
IHL
6 bits
16 bits
Type of Service
Total length
D
MF
F
Identification
TTL
2
protocol
Fragment offset
Header checksum
Source Add
Destination Add

options ( 0 or more words)
•
•
•
header has 20bytes fixed part and a variable length optional part.
Version keeps track of which version of the protocol the datagram belongs to
IHL – header length in 32 bit words. minimum IHL = 5 words = 20 bytes.
maximum is 15 words.
types of service – 6 bit field. voice, text etc. different classes of service.
total length – 65,535 bytes including both data and header.
•
•

•
•
•
•
•
•
•
•
•
•
Identification – identifies the fragment which datagram it belongs to.
An unused bit fields.
DF stands for Don’t Fragment.
MF stands for More Fragment – all fragments except the last one has this
bit set.
Fragment offset – where in the current datagram this fragment belongs.
there is a maximum 213 fragments per datagram.
TTL – counter to limit packet lifetimes in seconds. max. value 255. it must
be decremented on each hop, and supposed to be decremented when
queued for long time in a router. when it hits 0 the packet is discarded and a
warning message is sent back to source.
Protocol – TCP or UDP etc.
checksum – for header only.
source and destination address – IP addresses.
Options – security, strict source routing, loose source routing, record routes,
timestamp etc.
Classful addressing
A
0 network
B
10
C
110
D
1110
E
1111
1.0.0.0 –
127.255.255.255
Host
128.0.0.0 –
191.255.255.255
Host
network
network
Mutlicast address
For future use
192.68.1.20/255.255.255.0 or 192.168.1.20/24
network number or address – 192.68.1.0
host number of address – 20
Broadcast address = 198.68.1.255
Host
192.0.0.0 –
223.255.255.255
224.0.0.0 –
239.255.255.255
240.0.0.0 –
255.255.255.255
Special IP
• 0.0.0.0 – this host; used when the machine is
booting.
• IP address with 0 as the network number refer to
the current network.
• IP with all 1s allows broadcasting on local
network, typically a LAN.
• IP address with all 1s in host field allows
broadcasting on the remote network that
matches the network number.
• 127.x.x.x - loopback
Transporting a datagram
• Addressing and routing
• Key fields of IP datagram
A
223.1.2.2
223.1.1.4
223.1.2.9
Hub
223.1.1.3
Source IP Desti.IP
fields
address
Data
address
E
223.1.1.1
223.1.1.2
Misc
223.1.2.1
223.1.3.27
Dest. Net
Next
router
Nhop
s
223.1.1.0/24
-
1
223.1.2.0/24
223.1.1.
4
2
223.1.3.0/24
223.1.1.
4
2
Routing table in A
B
Dest. Net
Next router
Nhops
interface
223.1.1.0/24
-
1
223.1.1.4
223.1.2.0/24
-
1
223.1.2.9
1
223.1.3.27
223.1.3.0/24
Routing table in router
• How is this table created in the router?
• Internet corporation for Assigned Names and
Numbers (ICANN) – manages IP numbers
and DNS root servers
• ICANN appointed some regional authority
called Internet registry who are now
responsible for IP address assignment for a
particular region
– North america: American registry for internet
– Reseaux IP Europeans
– Asia pacific Network Information center (APNIC)
A campus network
hosts
routers
APE
CSE
main
rout
er
Subnets
• Splitting a network into several subnetworks.
– Reduced network traffic
• Routers create broadcast domains. The smaller broadcast
domains you create, the less network traffic on that network
segment.
– Optimized network performance
• This is a result of reduced network traffic
– Simplified management
• It’s easier to identify and isolate network problems in a group
of smaller connected networks than within one gigantic
network.
– Facilitated spanning of large geographical
distances
Subnetting example
• Let a class B network 130.50.0.0/16
1 0 0 0 0 0 1 0.0 0 1 1 0 0 1 0.0 0 0 0 0 0 0 0.0 0 0 0 0 0 0 0
16 bit
10
network
16 bit
subnet
host
11111111111111111111110000000000
Number of subnets = 26 = 64, new subnet mask = 255.255.252.0 = 22 bits
First subnet address – 130.50.0.0/22
2ndnd subnet address – 130.50.4.0/22
3rd subnet address – 130.50.8.0/22
Last subnet address – 130.50.252.0/22
CIDR
• Problem with classfull addressing:
• Practice of organizing the address space in classes wastes millions of
them. For most organization Class A with 16 million addresses is too
big, and a Class C with 256 addresses is too small. However, an
organization with 2000 hosts will also waste many IPs if it has a class B
network.
• Routing table explosion problem: more bits for net address makes larger
routing table.
• Classless InterDomain Routing
• Variable size block without regard to classes. If we need 2000
addresses we will be assigned 2048 address.
• The address is written as a.b.c.d/subnet mask in bits. 172.16.4.5/20
• Needs longest prefix match.
CIDR example
• We are to asked to create an ip block of 2048 addresses starting
with 194.24.0.0.
• 2048 = 11 bits host
• So 21 bits network.
11000010.00011000. 00000000. 00000000 =>194.24.0.0/21
00000000.00000000. 00000111. 11111111
=> Complement of the subnet mask
OR 11000010.00011000. 00000111.11111111 => 194.24.7.255 /21
Next block of 2048 start from 194.24.8.0/21 to 194.24.15.255/21
Next block of 2048 start from 194.24.16.0/21 to 194.24.23.255/21
However, if you have to allocate a block of 4096 ips to an university, then what
would be the range of ips and subnet mask?
Internet control protocols
• To send some control message, in addition to data,
network layer has control protocols like ICMP, ARP,
RARP, BOOTP and DHCP.
ICMP
• When an unexpected event occurs in the network, router
informs it by ICMP.
• Some important events and corresponding ICMP messages
are listed below:
Message type
Event description
Destination unreachable
Packet could not be delivered. DF bit set, small
packet network on the way.
Time exceeded
TTL field hit 0
Parameter problem
Invalid header field
Source quench
Choke packet. Receiver slow down the sending rate.
Redirect
Teach a router about geography. Packet seems to be
routed wrong.
Echo
Ask if the machine is alive
Echo reply
Yes, I am alive
Timestamp request
Same as echo but with timestamp
Timestamp reply
Same as echo reply, with timestamp
ARP
• Address Resolution Protocol
– used to get the physical address of a host.
– Ethernet physical address is called MAC address: 48 bits long.
e.g. 00.1c.3d.6f.23.a8
• How does the IP addresses get mapped onto physical
address?
– Source machines IP software sends a broadcast message asking
“who has IP 192.31.65.5? Tell 192.31.65.7” this is called ARP
broadcast.
– Only machine with ip 192.31.65.5 reply with its MAC address (say,
E2) to machine with IP 192.31.65.7
• ARP table – cache the address, age after a while
Explanation
• How does host 1 send a packet to host 4?
• ARP proxy
• Default router
RARP, BOOTP,DHCP
• Reverse Address Resolution Protocol.
– Used to get ip by a newly-booted machine.
– No more usage.
• Given an IP address what is the physical
address? Broadcast message.
• RARP broadcast is not routed.
• BOOTP
– Used to boot a diskless machine over the network.
– BOOTP messages are forwarded by the router.
– Manual entry of any new machine added in the
network.
DHCP
• Dynamic Host Configuration Protocol.
• Special server DHCP server is responsible for offering IP
against a MAC request over the network. Allows both
manual and automatic assignment.
• DHCP relay agent is needed on each LAN.
• DHCP DISCOVER (broadcast).
• Relay agent relays the discover message as unicast to
dhcp server possibly on a distant network.
• DHCPOFFER from the servers.
• DHCPREQUEST to the selected server.
• DHCPACK from the selected server.
• Leasing time.
Mobile IP
Problems: when a machine with an IP goes from one
network to another network, packets destined to it still
goes to its old network, and is lost.
1. assigning a new ip to a new host is not practical
because this would involve a huge information changes
in existing databases in the networks.
2. routers with complete IP addresses for routing, instead
of just the network; however, this strategy would require
each router to have millions of table entries! impractical.
IETF working group considers the following goals to be
achieved in any solution for the mobile net user:
1. each mobile host must be able to use its home ip
address anywhere.
2. software changes to the fixed hosts were not permitted.
3. changes to the router software and tables were not
permitted.
4. most packets for mobile hosts should not make detours
on the way.
5. no overhead should be incurred when a mobile host in
at home.
every site that wants to allow its user to roam has to create a
home agent. every site that wants to allow visitors has to
create a foreign agent. when a mobile host shows up at a
foreign site, it contacts the foreign agent there and
registers. the foreign agent then contacts the user’s home
agent and gives it a care of address, normally the foreign
agent’s won IP address.
when a packet arrives at the user’s home LAN, it comes in at
some router attached to the lan. the router then tries to
locate the host in the usual way, by broadcasting an ARP
packet asking, ethernet address of the host. the home
agent responds to this query by giving his its own ethernet
address. the router then sends the packet for the mobile
host to the home agent. It, in turn, tunnels then to the careof-address by encapsulating then in the payload field of an
iP packet addressed to the foreign agent.
the foreign agent then decapsulates and delivers them to the
data link address of the mobile host. In addition the home
agent gives the care of address to the sender, so future
packets can be tunneled directly to the foreign agent.